linux-kernel - Re: [PATCH v2] fs: add NULL check in drop

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Message-ID: <ujd4c2sadpu3fsux2t667ef3zz2i2nyiqvhes4ahbtpay4ba3f@unn3uh57fxdk>
Date: Wed, 10 Dec 2025 10:55:42 +0100
From: Jan Kara <jack@...e.cz>
To: Deepak Karn <dkarn@...hat.com>
Cc: Jan Kara <jack@...e.cz>, djwong@...nel.org, brauner@...nel.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	syzbot+e07658f51ca22ab65b4e@...kaller.appspotmail.com, syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk, 
	David Howells <dhowells@...hat.com>, linux-afs@...ts.infradead.org
Subject: Re: [PATCH v2] fs: add NULL check in drop_buffers() to prevent
 null-ptr-deref

On Tue 09-12-25 22:00:04, Deepak Karn wrote:
> On Tue, Dec 9, 2025 at 4:48 PM Jan Kara <jack@...e.cz> wrote:
> >
> > On Tue 09-12-25 01:43:33, Deepakkumar Karn wrote:
> > > On Mon, 8 Dec 2025 11:30:24 -0800, Darrick J. Wong wrote:
> > > > > drop_buffers() dereferences the buffer_head pointer returned by
> > > > > folio_buffers() without checking for NULL. This leads to a null pointer
> > > > > dereference when called from try_to_free_buffers() on a folio with no
> > > > > buffers attached. This happens when filemap_release_folio() is called on
> > > > > a folio belonging to a mapping with AS_RELEASE_ALWAYS set but without
> > > > > release_folio address_space operation defined. In such case,
> > >
> > > > What user is that?  All the users of AS_RELEASE_ALWAYS in 6.18 appear to
> > > > supply a ->release_folio.  Is this some new thing in 6.19?
> > >
> > > AFS directories SET AS_RELEASE_ALWAYS but have not .release_folio.
> >
> > AFAICS AFS sets AS_RELEASE_ALWAYS only for symlinks but not for
> > directories? Anyway I agree AFS symlinks will have AS_RELEASE_ALWAYS but no
> > .release_folio callback. And this looks like a bug in AFS because AFAICT
> > there's no point in setting AS_RELEASE_ALWAYS when you don't have
> > .release_folio callback. Added relevant people to CC.
> >
> >                                                                 Honza
> 
> Thank you for your response Jan. As you suggested, the bug is in AFS.
> Can we include this current defensive check in drop_buffers() and I can submit
> another patch to handle that bug of AFS we discussed?

I'm not strongly opposed to that (although try_to_free_buffers() would seem
like a tad bit better place) but overall I don't think it's a great idea as
it would hide bugs. But perhaps with WARN_ON_ONCE() (to catch sloppy
programming) it would be a sensible hardening.

Also I think mapping_set_release_always() should assert that
mapping->a_ops->release_folio is non-NULL to catch the problem early (once
you fix the AFS bug).

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR